Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2

A key aspect of unconventional pairing by the antiferromagnetic spin-fluctuation mechanism is that the superconducting energy gap must have opposite sign on different parts of the Fermi surface. Recent observations of non-nodal gap structure in the heavy-fermion superconductor CeCu$_2$Si$_2$ were then very surprising, given that this material has long been considered a prototypical example of a superconductor where the Cooper pairing is magnetically mediated. Here we present a study of the effect of controlled point defects, introduced by electron irradiation, on the temperature-dependent magnetic penetration depth $\lambda(T)$ in CeCu$_2$Si$_2$. We find that the fully-gapped state is robust against disorder, demonstrating that low-energy bound states, expected for sign-changing gap structures, are not induced by nonmagnetic impurities. This provides bulk evidence for $s_{++}$-wave superconductivity without sign reversal.Comment: 5 pages, 4 figures + Supplemental Material (1 page, 1 figure). Will appear in Phys. Rev. Let

Correlation between Fermi surface transformations and superconductivity in the electron-doped high-TcT_c superconductor Nd2−x_{2-x}Cex_xCuO4_4

Two critical points have been revealed in the normal-state phase diagram of the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_4$ by exploring the Fermi surface properties of high quality single crystals by high-field magnetotransport. First, the quantitative analysis of the Shubnikov-de Haas effect shows that the weak superlattice potential responsible for the Fermi surface reconstruction in the overdoped regime extrapolates to zero at the doping level $x_c = 0.175$ corresponding to the onset of superconductivity. Second, the high-field Hall coefficient exhibits a sharp drop right below optimal doping $x_{\mathrm{opt}} = 0.145$ where the superconducting transition temperature is maximum. This drop is most likely caused by the onset of long-range antiferromagnetic ordering. Thus, the superconducting dome appears to be pinned by two critical points to the normal state phase diagram.Comment: 9 pages; 7 figures; 1 tabl

Controlling crystal cleavage in Focused Ion Beam shaped specimens for surface spectroscopy

Our understanding of quantum materials is commonly based on precise determinations of their electronic spectrum by spectroscopic means, most notably angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). Both require atomically clean and flat crystal surfaces which traditionally are prepared by in-situ mechanical cleaving in ultrahigh vacuum chambers. We present a new approach that addresses three main issues of the current state-of-the-art methods: 1) Cleaving is a highly stochastic and thus inefficient process; 2) Fracture processes are governed by the bonds in a bulk crystal, and many materials and surfaces simply do not cleave; 3) The location of the cleave is random, preventing data collection at specified regions of interest. Our new workflow is based on Focused Ion Beam (FIB) machining of micro-stress lenses in which shape (rather than crystalline) anisotropy dictates the plane of cleavage, which can be placed at a specific target layer. As proof-of-principle we show ARPES results from micro-cleaves of Sr$_2$RuO$_4$ along the ac plane and from two surface orientations of SrTiO$_3$, a notoriously difficult to cleave cubic perovskite

Strain tuning in microstructured quantum materials F

The application of strain to quantum materials is a powerful technique for tuning electronic correla-tions and the balance between interaction parameters by favoring specific electronic phases over al-most degenerate competing orders via breaking underlying crystal symmetries. For example, it can promote a long-range charge-ordered state over high-temperature superconductivity in cuprates [1,2] or induce a chiral state in Kagome metals [3]. To maximize surface strains, we exploit the enhanced yield strain of micron-scale materials, well-stud-ied in materials science [4]. State-of-the-art microfabrication using focused ion beam techniques allow precise design of crystalline samples, achieving desired strain fields such as uniaxial stress or more complex strain gradients [5]. We microcarve the entire sample into a flexible cantilever without a sub-strate and then bend it, enabling arbitrary and especially out-of-plane tensile strain even in layered quantum materials [6]. Raman scattering directly probes long-wavelength phonon modes, which are highly sensitive to lattice strain. With submicrometer spatial resolution, it provides a direct measure of strain variations. It also detects local symmetry breaking and gives access to electronic, magnetic, and orbital excitations, prob-ing the electronic ground state. Even without a change in lattice symmetry under stress, the phonon mode frequency serves as an extremely sensitive probe, determined with high energy resolution. The layered crystal structure of delafossite PdCoO2, with weak interlayer coupling, makes it an ideal candidate for studying out-of-plane tensile strain. Its exceptional purity [7] minimizes extrinsic disorder effects. Among the various Raman modes allowed by group theory, the fully symmetric A1g phonon, consisting of oxygen ion vibrations along the c-direction [8], is particularly interesting. Finite element simulations guide the design of a cantilever manufactured from high-quality single crystals of PdCoO2. Together with DFT-calculations our Micro-Raman measurements confirm quantitively the spatial strain distribution on the cantilever. Furthermore, we investigate the role of the amorphous layer thickness for Raman spectra. [1] Kim, H. H. et al. Uniaxial pressure control of competing orders in a high-temperature superconductor. Science 362, 1040-1044, doi:10.1126/science.aat4708 (2018). [2] Kim, H. H. et al. Charge Density Waves in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.67}$ Probed by Resonant X-Ray Scattering under Uniaxial Compression. Physical Review Letters 126, 037002, doi:10.1103/PhysRevLett.126.037002 (2021). [3] Guo, C. et al. Switchable chiral transport in charge-ordered kagome metal CsV3Sb5. Nature 611, 461-466, doi:10.1038/s41586-022-05127-9 (2022)

Observation of the Non-linear Meissner Effect

A long-standing theoretical prediction is that in clean, nodal unconventional superconductors the magnetic penetration depth $\lambda$, at zero temperature, varies linearly with magnetic field. This non-linear Meissner effect is an equally important manifestation of the nodal state as the well studied linear-in-$T$ dependence of $\lambda$, but has never been convincingly experimentally observed. Here we present measurements of the nodal superconductors CeCoIn$_5$ and LaFePO which clearly show this non-linear Meissner effect. We further show how the effect of a small dc magnetic field on $\lambda(T)$ can be used to distinguish gap nodes from non-nodal deep gap minima. Our measurements of KFe$_2$As$_2$ suggest that this material has such a non-nodal state